1. Field of the Invention
This invention relates to a fuel feed control system suitable for use with an internal combustion engine mounted on an automotive vehicle, and especially to a fuel feel control system which, even when the state of operation of the engine is in a transition period, permits setting of an accurate injection quantity of fuel by precisely measuring a quantity of inducted air.
2. Description of the Related Art
In an internal combustion engine equipped with fuel injectors, the quantity of inducted air is generally detected, for example, by an air flow sensor and the quantity of fuel to be injected is then determined in accordance with the quantity of inducted air, whereby the air/fuel ratio (A/F) is controlled.
Since the injection of fuel in a 4-cycle engine is generally performed before the end of each induction stroke, information on the quantity of inducted air (A/N: the quantity of air inducted per revolution of a crankshaft of the engine) to determine the quantity of fuel to be injected is sampled at a time before the actual induction stroke. Described specifically, the quantity of fuel is determined, as shown in FIG. 9, based on a detected value of the quantity of air inducted about two strokes before the end of induction of air at which a real quantity of inducted air is determined.
FIG. 10 is a graph showing a relationship between detected values of inducted air quantities and real quantities of inducted air. In the graph, a line a represents A/N ratios detected by an air flow sensor. The point designated by each dot shows the value of an A/N ratio estimated from the pressure of inducted air when an engine is operated in a steady state, and indicates a real quantity of inducted air without any detection lag or the like.
As is shown in this graph, it is appreciated that each A/N ratio detected by the air flow sensor (line a) is shifted from the corresponding real A/N value indicated by a dot.
In the graph, a line b has been drawn by advancing by two strokes (360.degree.) the results of the detected A/N values indicated by the line a. When the line a is advanced by two strokes as indicated by the line b, the detected A/N values coincide with the corresponding real A/N values. In other words, the detection of each quantity of inducted air is delayed by two strokes. Incidentally, a line c indicates openings of a throttle valve.
If the quantity of fuel to be injected is determined based on the detected value of the quantity of air inducted two strokes before, the difference between the real quantity of inducted air and the quantity of inducted air detected based on the preceding sampling becomes greater during a transition period of operation state of the engine. This means, for example, that a real quantity of inducted air becomes greater than a corresponding detected value upon acceleration but the real quantity of inducted air becomes smaller than the corresponding detected value upon deceleration.
For accurate air/fuel ratio control, it is an essential requirement to detect a real quantity of inducted air accurately without a delay. In actuality, however, it is very difficult to avoid a lag in the detection of a quantity of inducted air because fuel is injected before induction of air into a combustion chamber is completed.
Known as a method for compensating this detection lag is the differential predictive correction method which makes use of the difference between a latest detection value and a preceding detection value. A calculation formula for a quantity of inducted air by the differential predictive correction method can be illustrated as shown below.
A/NF(n)=A/N(n)+m.multidot.[A/N(n)-A/N(n-1)]
where
A/NF(n): Predicted value in the current control. PA0 A/N(n): Detected value of the quantity of inducted air in the current control. PA0 A/N(n-1): Detected value of the quantity of inducted air in the preceding control. PA0 m: Prediction gain.
Here, the prediction gain m is defined as a constant which is determined by the difference in timing between the detection and the end of the induction stroke (m is 1 to 2 usually).
It is, however, rare that upon actual acceleration of an engine, the quantity of inducted air linearly increases as shown in FIG. 11A. Where the engine is accelerated causing a non-linear increase in inducted air such as shown by the non-linear line in FIG. 11B, use of a fixed value as the prediction gain m tends to result in insufficient prediction especially in an initial stage of acceleration.
However any attempt to positively perform a correction by increasing the prediction gain m then leads to amplification of minute changes in the detected value of the quantity of inducted air during steady state operation so that the quantity of fuel fluctuates. This results in the potential problem that the A/F ratio varies to induce misfire or fluctuations in torque.
Examples of techniques for making it possible to change the prediction gain include the technique disclosed in Japanese Patent Publication (Kokoku) No. HEI 4-19377. This technique is to set a gain on the basis of the state of operation of an engine by determining whether or not the operation state of the engine is an idling state. When the operation state of the engine is determined to be an idling state, the gain is set at a predetermined value .phi..sub.1 conforming with characteristics of the idling state of the engine. When the operation state of the engine is not determined to be an idling state, the gain is then set at another predetermined value .phi..sub.2.
In a fuel feed control system designed to permit changing of the prediction gain as mentioned above, however, the prediction gain for the estimation of a quantity of inducted air is set only at two values, one being a gain (the predetermined value .phi..sub.1) for an idling state and the other a gain (the predetermined value .phi..sub.2) for operation states other than idling. The fuel feed control system is therefore accompanied by the problem that, during normal operation other than idling, the gain is fixed at the predetermined value .phi..sub.1 and changes in A/F, torque and the like cannot be reduced surely.